Roberta Aguirre
Alternate airport requirements are a set of criteria that determine whether an aircraft needs to file for an alternate airport. This is necessary when weather conditions are poor. Aircraft must always file for an alternate airport unless the airport has an instrument approach, and weather forecasts indicate that one hour before and after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation with a visibility range of at least 3 statute miles. These requirements vary based on aircraft type and approach category, with different rules for helicopters and planes with different engine configurations.
Alternate Airport Requirements
| Characteristics | Values |
|---|---|
| Definition | An alternate airport is an airport to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the airport of intended landing. |
| Requirements | An alternate airport is required if it is a yes to the 1-2-3 rule. |
| Destination Alternate | A destination alternate is an aerodrome at which an aircraft would be able to land should it become either impossible or inadvisable to land at the aerodrome of intended landing. |
| Take-off Alternate | A take-off alternate aerodrome should be specified in the operational flight plan if either the meteorological conditions at the departure aerodrome are below the applicable landing minima or in case it should not be possible to return to the departure aerodrome for some other reason. |
| En-route Alternate | An en-route alternate is an aerodrome at which an aircraft would be able to land in the event that a diversion becomes necessary while en route. |
| Fuel Requirements | Day or night, you must have enough fuel to: Fly to your intended destination, Fly from that airport to the alternate airport, and Fly after that for 45 minutes at normal cruising speed or, for helicopters, fly for 30 minutes. |
| Weather Conditions | For aircraft other than helicopters, for at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles. |
| Personal Minimums | You should always factor in personal minimums. If you're not comfortable flying into low conditions, plan a better alternative. |
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What You'll Learn
Requirements for different aircraft
Alternate airport requirements vary depending on the type of aircraft and the flight plan. Here are the requirements for different types of aircraft:
IFR Aircraft
For Instrument Flight Rules (IFR) aircraft, the requirements for alternate airports are more stringent due to the potential for reduced visibility and other factors. The 1-2-3 rule is commonly applied, which states that an alternate airport is required if, within one hour before and after the estimated time of arrival, the ceiling is forecast to be below 2,000 feet or visibility is less than 3 statute miles. This rule ensures that pilots have sufficient visibility to navigate safely.
IFR aircraft must also consider the equipment on board and its compatibility with the airport's instrument approach procedures. For example, older IFR-approach GPS receivers may only be approved for LNAV-only approaches, requiring verification of Receiver Autonomous Integrity Monitoring (RAIM).
VFR Aircraft
Visual Flight Rules (VFR) aircraft have more flexibility regarding alternate airports. While it is not a legal requirement to declare an official alternate airport, VFR pilots should always have a few potential airports in mind along their route and near their destination. This contingency planning is crucial, as VFR flights can encounter unexpected weather changes or other issues that necessitate diverting to an alternate airport.
Helicopters
Helicopters have unique alternate airport requirements. Per 14 CFR 91.169, helicopters are not mandated to list an alternate airport in their IFR flight plan if specific conditions are met. These conditions include a ceiling of at least 2,000 feet above the airport elevation and visibility of at least 3 statute miles for one hour before and after the estimated time of arrival.
Multi-Engine Aircraft
Multi-engine aircraft, particularly those with two engines, have specific alternate airport requirements. In the event of an engine failure, these aircraft must be able to reach a suitable alternate airport within one hour's flight time at the one-engine-inoperative cruising speed specified in the Aerodrome Operating Manual (AOM). This requirement ensures that multi-engine aircraft can safely divert to an alternate destination if necessary.
Take-off and En-route Alternates
Take-off alternates are aerodromes where an aircraft can land shortly after take-off if returning to the departure aerodrome is not possible. These are especially important when the departure aerodrome's meteorological conditions are below landing minima. En-route alternates, on the other hand, are aerodromes where an aircraft can land if a diversion becomes necessary while in transit. These are typically required for specific circumstances and may be the same as the departure or destination aerodrome.
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Terrain considerations
Firstly, it is crucial to assess the impact of terrain on weather conditions. Certain terrain features, such as mountains or hills, can influence wind patterns, cloud formation, and visibility. For example, an airport nestled in a valley between mountain ranges can have different weather patterns compared to one located on higher ground. Understanding how terrain affects weather is essential for making informed decisions about alternate airports.
Secondly, when considering terrain, it is important to evaluate the highest obstacles and terrain features surrounding the airport. This includes identifying obstacles such as smokestacks, towers, or antennas depicted on sectional charts. By knowing the elevation of these obstacles, pilots can ensure safe clearance during approach and departure procedures. Contour lines on charts can also provide valuable information about the terrain's elevation and slope, helping pilots anticipate potential challenges or restrictions during flight operations.
Additionally, running water, such as a river, can serve as an indicator of terrain characteristics. By observing the direction of water flow, pilots can deduce the general slope and gradient of the surrounding terrain. A river with a lazy straight line suggests a gentler terrain slope, while a river that switches back and forth indicates rougher terrain with varied surface conditions.
Moreover, it is worth noting that terrain considerations may result in nonstandard alternate airport requirements. These requirements can vary depending on the aircraft's approach category. Some airports might not be available as alternates due to the absence of on-airport weather reporting capabilities, which are essential for safe navigation and decision-making.
Lastly, when dealing with congested areas, it is important to adhere to conservative interpretations of this term. The FAA considers a congested area to include groups of people on airport ramps, beachgoers, subdivisions, and even traffic on highways. This definition underscores the importance of selecting alternate airports that offer adequate clearance from potential obstacles and populated areas.
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Weather conditions
For IFR flights, pilots are required to file an IFR flight plan and receive an ATC clearance. However, the actual weather conditions during the flight may vary, and only a handful of flights each year may require an instrument approach at the destination. It is recommended to file an IFR flight plan, especially for cross-country trips, to take advantage of the ATC services provided. The IFR flight plan includes an "Alternate Airport" field, which should be carefully considered based on the weather conditions.
FAR 91.169 outlines two scenarios regarding weather conditions and alternate airport requirements:
- If the weather at the destination, for at least one hour before and one hour after the estimated time of arrival, is forecast to have a ceiling of at least 2,000 feet above the airport elevation and visibility of at least three statute miles, an alternate airport is not required in the IFR flight plan.
- If the forecasted conditions at the destination, within one hour before and one hour after the estimated time of arrival, indicate a ceiling below 2,000 feet or visibility of less than three statute miles, an alternate airport must be filed.
When choosing an alternate airport, it is crucial to consider the weather conditions at the time of arrival. The standard alternate minimums, as per 14 CFR 91.169, specify the following:
- For a precision approach procedure: a ceiling of 600 feet and visibility of two statute miles.
- For a non-precision approach procedure: a ceiling of 800 feet and visibility of two statute miles.
It is important to note that some airports may have non-standard alternate minimums due to terrain considerations, and these requirements may vary by aircraft approach category. Additionally, airports without weather reporting capabilities or with unmonitored approach equipment may not be suitable as alternates.
In summary, weather conditions are a key factor in determining alternate airport requirements. Pilots should carefully consider the weather forecasts and regulations to make informed decisions and ensure a safe flight.
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Time en route
When it comes to time en route, the requirements for alternate airports vary depending on the type of flight and the number of engines the aircraft has.
For domestic US flights, an alternate airport is not required unless the weather conditions at the destination are below specific minimums. In this case, the "1-2-3 rule" applies. This means that if the forecasted conditions at the destination are expected to have a ceiling below 2,000 feet or visibility of less than 3 statute miles within one hour before and after the estimated time of arrival, an alternate airport must be listed in the IFR flight plan.
For international flights, the regulations depend on the rules of the countries whose airspace is being used. In some cases, an alternate airport may not be required if there is an instrument approach procedure available at the destination, and the forecasted weather meets certain minimums.
Take-off and en-route alternates are typically only required in specific circumstances. A take-off alternate aerodrome should be specified in the operational flight plan if the meteorological conditions at the departure aerodrome are below the applicable landing minima or if it is not possible to return to the departure aerodrome for some other reason. For aeroplanes with two engines, the take-off alternate must be within one hour's flight time at the one-engine-inoperative cruising speed, as specified in the Aerodrome Operating Manual (AOM). Aeroplanes with three or more engines are permitted two hours of flight time under the same conditions, except at the all-engines-operating cruising speed. An en-route alternate aerodrome is specifically required for EDTO operations by aeroplanes with two turbine engines, and it must be included in the operational and ATS flight plans.
When choosing an alternate airport, it is important to consider the time and fuel required to divert to the alternate airport. Tools like ForeFlight or Garmin Pilot can assist in this process by providing information on the time and fuel needed to divert to potential alternate airports.
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Safety risk assessments
When conducting safety risk assessments for alternate airports, it is essential to consider various factors and regulations. Firstly, the overall weather trends in the vicinity should be analysed. This involves examining the weather conditions at the destination and alternate airports. Legal alternate minimums, such as the 3-2-1 rule, should be considered, ensuring that weather conditions meet the required standards, including ceiling height and visibility. For instance, the 3-2-1 rule dictates that unless an airport has an instrument approach, a flight plan must be filed with an alternate airport unless, one hour before and after the ETA, the ceiling is at least 2,000' AGL and visibility is at least 3 SM.
In addition to weather conditions, other factors come into play when selecting an alternate airport. These include the availability of necessary services and facilities, ensuring that aircraft performance requirements can be met, and confirming the airport's operational status at the expected time of use. The type of aircraft and the number of engines should also be considered, as this affects the allowable flight time to the alternate airport.
Furthermore, safety risk assessments should take into account personal minimums and comfort levels. Pilots should assess their own capabilities and plan accordingly, especially when flying into low-condition areas. Having a well-thought-out "Plan B" is crucial, and factors such as fuel availability, runway adequacy, and proximity to urban areas can make an alternate airport a more favourable choice.
It is important to note that certain circumstances may require specific types of alternate airports, such as Take-off, En-route, or Destination alternates. A Take-off alternate is necessary when it is impossible to use the departure aerodrome shortly after takeoff. An En-route alternate is designated when a diversion becomes necessary while en route, and a Destination alternate is required when it becomes impossible or inadvisable to land at the intended airport.
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Frequently asked questions
An alternate airport is an airport to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or land at the intended airport.
You need to file an alternate airport unless the airport has an instrument approach and weather reports and forecasts indicate that 1 hour before and after your ETA, the ceiling will be at least 2,000' AGL and the visibility will be at least 3 SM.
Some factors to consider are the weather conditions, the distance from the destination, the availability of fuel, and the proximity to an urban area.
